1. Field of the Invention
The present invention relates to a liquid crystal display (hereinafter referred to as an “LCD”) and a method for manufacturing the LCD and more particularly to the LCD in which an alignment layer is formed in a manner to cover a TFT (Thin Film Transistor) in a TFT substrate and to the method for manufacturing the LCD.
The present application claims priority of Japanese Patent Application No. 2000-275708 filed on Sep. 11, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
An LCD is widely used as a display device for various information systems or a like. The LCD is so constructed that a liquid crystal is put or injected between a TFT substrate in which a TFT operating as a switching element is formed and a facing substrate. Such the LCD is roughly classified into a TN (Twisted Nematic)-LCD and an IPS (In-Plane Switching)-LCD according to a difference in a display method.
The TN-LCD is so configured that a pixel electrode is mounted on the TFT substrate and a common electrode on the facing substrate and that a driving voltage is applied to both the electrodes to cause a longitudinal electric field to be produced relative to the TFT substrate for operations. On the other hand, the IPS-LCD is so configured that both the pixel electrode and common electrode are mounted on one of the substrates, for example, on the side of the TFT substrate in a manner that both the pixel electrode and common electrode are placed in a direction opposite to each other in one plane and that a driving voltage is applied to both the electrodes to cause a traverse electric field to be produced relative to the TFT substrate for operations. Since the IPS-LCD exhibits an orientation of liquid crystal molecules along a surface of the TFT substrate, it has an advantage in that it can provide a wider viewing angle compared with the TN-LCD. Therefore, there is a tendency that the IPS-LCD is preferably used.
FIG. 28 is a top view of a conventional LCD. FIG. 29 is a cross-sectional view of the conventional LCD of FIG. 28 taken along the line N—N. FIG. 30 is a top view showing configurations of components existing in a vicinity of a gate electrode making up a main part of the conventional LCD of FIG. 29. FIG. 31 is a cross-sectional view of the conventional LCD of FIG. 30 taken along the line O—O. In FIG. 31, only a TFT substrate 51 is shown. As shown in FIG. 28 to FIG. 31, the conventional LCD is so configured that a liquid crystal 53 is sandwiched between the TFT substrate 51 and a facing substrate 52. The TFT substrate 51 includes a first transparent substrate 54 made of glass or a like, a first polarizer 55 formed on an underside of the first transparent substrate 54, a gate electrode 56 made of Al, Cr, Mo, or a like formed on a part of a surface of the first transparent substrate 54, an interlayer dielectric 57 made up of stacked films composed of SiO2 (silicon dioxide) and SiN (silicon nitride) formed on the gate electrode 56 in a manner to cover the gate electrode 56, a semiconductor layer 58 consisting of a-Si (amorphous silicon) film or a like formed on the interlayer dielectric 57, ohmic layers 59A and 59B made of an n+ type a-Si film or a like each being formed on each of end portions of the semiconductor layer 58, a drain electrode 61 made of Cr, Mo, or a like in a manner to be connected to the ohmic layer 59B, a source electrode 62 made of Cr, Mo, or a like in a manner to be connected to the ohmic layer 59A, a passivation film 63 made of the SiN or a like grown in a manner to cover the drain electrode 61, source electrode 62, and semiconductor layer 58; and a first alignment layer 64 made of polyimide or a like formed in a manner to cover the passivation film 63.
As shown in FIG. 28 and FIG. 30, in the TFT substrate 51, a data line 73 is formed in a direction orthogonal to the gate electrode 56 (that is, in a direction of a length of the TFT substrate 51) in a manner to be connected to the drain electrode 61 and a pixel electrode 74 is formed in a manner to be connected to the source electrode 62. The pixel electrode 74 and source electrode 62 are formed by using the same material and in a same process. Moreover, a common electrode 75 is formed in a manner to be placed opposite to the pixel electrode 74 in one plane. The common electrode 75 and gate electrode 56 are formed by using a same material and in a same process. Rubbing processing is performed in a rubbing direction 76 on the first alignment layer 64 on the TFT substrate 51. In order to determine a rotational direction of the liquid crystal 53 injected between the TFT substrate 51 and facing substrate 52, the rubbing direction 76 is set so as to be inclined by a fixed angle from the longitudinal direction.
The facing substrate 52 making up the conventional LCD includes a second transparent substrate 66 made of glass or a like, a second polarizer 58 formed on a back of the second transparent substrate 66 through a conductive layer 67, a black matrix 69 made of Ti, Cr, carbon resin, or a like formed on a surface of the second transparent substrate 66, a color layer 70 formed in a manner to cover the black matrix 69 and a second alignment layer 72 made of polyimide or a like formed on a planarized film 71 in a manner to cover the color layer 70.
The rubbing processing is performed by using a rubbing roller 80 as shown in FIG. 32. That is, a rubbing bristle 79 is attached through rubbing cloth 78 to the rubbing roller 80. By rotating the rubbing roller 80 and moving the TFT substrate 51 with the first alignment layer 64 mounted thereon under the rubbing roller 80 that is rotating, surfaces of the alignment layer 64 are rubbed by the rubbing fur 79 and, as a result, rubbing trenches are formed. However, at a time of the rubbing processing, shavings 81 of the alignment layer 64 are produced. If the shavings 81 are left on the pixel electrode 74, orientation of the liquid crystal 53 is perturbed, causing a display defect. To solve this problem, generally, in order to remove such the shavings 81 of the alignment layer 64, rubbing washing is performed by splashing pure water on the surface of the TFT substrate 51. However, though the shavings 81 of the alignment layer 64 can be removed from the pixel electrode 74 by such the rubbing washing, it is difficult to completely remove the shavings 81 being adhered to step portions of the TFT. That is, as shown in FIG. 31, the shavings 81 of the alignment layer 64 are still left at the step portions 83 of the TFT 82.
In the case of the IPS-LCD in particular, a material having low liquid crystal resistance is used to prevent display irregularity caused by accumulation of charges on the color layer 70, unwanted charge is induced in a back channel 84 of the TFT 82, which causes image retention when the liquid crystal 53 is used for a long time driving and displaying. To prevent the image retention as described above, it is necessary to eliminate an influence by the liquid crystal 53. To eliminate the influence by the liquid crystal 53, it is necessary to enhance insulation between the liquid crystal 53 and the back channel 84 of the TFT 82. More particularly, it is necessary to make large a thickness of the passivation film 63 to be grown on the back channel 84.
However, to make large the thickness of the passivation film 63, long deposition time has to be given in deposition process of the passivation film 63, which causes an increase in costs from a viewpoint of production and therefore which is impossible to be realized actually.
The inventor of the present invention has found that the shavings 81 of the alignment layer 64 being left on the step portions 83 on the back channel 84 of the TFT 82 can be effectively utilized as part of the insulating film serving to enhance the insulation between the liquid crystal 53 and back channel 84 of the TFT 82.
FIG. 33 is a result from an experiment carried out by the inventor of the present invention showing a relation among a rubbing pushing depth (abscissa), an alignment layer peeling level (abscissa on a left side) and image retention (abscissa on a right side). The rubbing pushing depth represents a cutting depth of the alignment layer 64 at the time of the rubbing processing using the rubbing roller 80. The alignment layer peeling level represents a degree of the occurrence of shavings 81 of the alignment layer 64. The experiment was carried out under conditions, for example, of a moving speed being 30 mm/s, the number of rotations being 1000 RPM (Revolutions Per Minute), a material for rotation roller being rayon or a like.
As is apparent from FIG. 33, there is a trade-off between the alignment layer peeling level and the image retention against the rubbing pushing depth. That is, the larger the rubbing pushing depth becomes, the worse the alignment layer peeling level and the better a degree of the image retention becomes. In contrast, the smaller the rubbing pushing depth becomes, the better the alignment layer peeling level and the worse the degree of the image retention. For example, if the rubbing pushing depth becomes about 0.8 mm, the alignment layer peeling level becomes a maximum 4 and, as a result, the shavings 81 of the alignment layer 64 are accumulated in the step portions 83 of the TFT 82 on the back channel 84, thus improving the degree of the image retention by about 0.5 points. This suggests, therefore, that, in order to improve the degree of the image retention, it is necessary to make larger the rubbing pushing depth.
However, the conventional LCD has a problem. That is, since the shavings 81 of the alignment layer 64 being effective for preventing the image retention are not accumulated much in the step portions 83 of the TFT 82 on the TFT substrate 51, the prevention of the image retention is difficult. That is, in the conventional LCD, as shown in FIG. 30 and FIG. 31, since the shavings 81 are removed by the washing processing following the rubbing processing on the alignment layer 64, the shavings 81 of the alignment layer 64 are not accumulated much in the step portions 83 of the TFT 82 on the back channel 84 in the TFT substrate 51. Therefore, the shavings 81 of the alignment layer 64 that are left hardly contribute to enhance the insulation between the liquid crystal 53 and the back channel 84 of the TFT 82.